A. Technical Field
The present invention relates to a microelectromechanical structure, and more particularly, to systems, devices and methods of incorporating z-axis out-of-plane stoppers that are controlled to protect the structure from mechanical shock with negligible electrostatic disturbance. This microelectromechanical structure is less susceptible to mechanical shock, and shows a negligible offset that may be induced by an electrostatic force imposed by a shock stopper.
B. Background of the Invention
A microelectromechanical structure (MEMS) is widely applied as a sensor to measure acceleration, rotation, pressure and many other physical parameters. The MEMS device is normally formed on a silicon substrate using a micromachining process, and thus, adopts characteristic feature sizes of several micrometers. Such miniaturized devices transduce mechanical movement to electrical signals that may indicate the level of the interested parameter. One typical MEMS device is a micro-machined capacitive accelerometer that comprises a proof mass. The proof mass is suspended above the silicon substrate, and anchored via elastic elements that are configured to offer desirable mechanical displacement or vibration. Various MEMS devices have been widely employed in applications ranging from consumer products to specialized products used under extreme environments, and nowadays, they may be easily found in automotive parts, mobile phones, gaming devices, medical appliance, and military applications.
Any MEMS device has to maintain an acceptable level of shock robustness in order for successful commercialization. The MEMS device is basically reliant on the mechanical response of its microstructure, and such structure is inevitably susceptible to mechanical vibration and shock coupled from the surrounding environments. However, shock is one main environmental influence that could potentially degrade device performance and cause permanent structural damage. If the MEMS device does not have sufficient immunity against the shock that may be experienced in its application, reliability is easily compromised, and failure could be expected to arise sooner or later.
Shock takes a form of an impulse-like mechanical load that develops from a large force within a short duration. According to its source, the magnitude of the shock may vary dramatically, ranging from 1 g in a free-fall hit to the ground to 10,000 g or larger under some extreme conditions. A higher shock level requires the MEMS device to incorporate more sophisticated anti-shock techniques, and normally induces a higher manufacturing cost. It is a waste to design the MEMS device to be excessively robust, when its application does not impose so. In general, the MEMS device has to be engineered according to the particular application that it is intended for, such that it may provide a reasonable level of shock robustness at a reasonable cost.
In some prior arts, the elastic elements are engineered to increase robustness of the MEMS device such that it may sustain a large shock load. A special medium may also be used to fill the package that encloses the MEMS device, and the shock load is significantly dampened prior to reaching the device. However, these techniques impact the structure directly, and compromises device performance under normal operational conditions as well. A need exists to protect the MEMS device from the shock while maintaining its normal device performance.